I've heard that it's impossible to sell a book longer than 100,000 words. Is this true?
There's some truth to the fact that the longer your manuscript, the more challenging it will be to sell it. Publishers often prefer novels that are the 80,000- to 100,000-word range, depending on the genre. Once you start getting into the 120,000- or 150,000-word range (hopefully you're not at 200,000!), an editor or agent may think twice about making an investment of time/energy in the work.
Word count becomes even more critical when writing in a genre with rigid conventions, such as category romance. Always check submission guidelines of book publishers (when submitting directly), or a resource like WritersMarket.com, to see if your book fits their specifications or preferences. Never consider your book so good that it would be an exception to the rule.